1. Field of the Invention
The present invention relates to a method of isolating single-walled carbon nanotubes. More particularly, the present invention relates to a method of isolating semiconducting carbon nanotubes having a specific chirality.
2. Description of the Related Art
Carbon nanotubes have anisotropic structures with various shapes, e.g., single-walled, multi-walled, and rope-like, which range in diameter from several nanometers to several tens of nanometers and range in length from several tens of microns to several hundreds of microns. Carbon nanotubes exhibit conductive or semiconductive properties depending on their chirality. Carbon nanotube powders contain a mixture of semiconducting carbon nanotubes and metallic carbon nanotubes. When carbon nanotubes have an arm-chair structure, they exhibit metallic properties, and when they have a zig-zag structure, they exhibit semiconductive properties. A semiconducting carbon nanotube has a quasi one-dimensional structure and an energy gap varying depending on its diameter, thus exhibiting a unique quantum effect.
Carbon nanotubes, which are hollow, are mechanically strong (about 100 times stronger than steel), chemically stable, and have high thermal conductivity. Thus, carbon nanotubes have been highlighted as a new functional material expected to have many microscopic and macroscopic applications. Extensive research has been conducted into using carbon nanotubes in various applications, e.g., in memory devices, electron amplifiers, gas sensors, microwave shields, electrode pole plates in electrochemical storage units (secondary batteries, fuel cells, or super capacitors), field emission displays (FEDs), and polymer composites.
To make these applications of carbon nanotubes practical, it is necessary to obtain carbon nanotubes having a specific chirality. For example, semiconducting carbon nanotubes are used in memory devices, sensors, etc., and metallic carbon nanotubes are used in electrode materials of cells, electromagnetic shields, etc. Thus, there is a need for a method of selectively producing carbon nanotubes having a specific chirality, or a method of isolating carbon nanotubes having a specific chirality from a group of carbon nanotubes.
Carbon nanotubes are commonly produced by chemical vapor deposition (CVD). However, it is difficult to selectively produce carbon nanotubes having a desired chirality using CVD. Methods of producing carbon nanotubes having specific chirality using electric discharging or laser deposition have been proposed. However, these methods have low yields. In addition, the use of a laser increases the cost of production, and perfect selective growth of carbon nanotubes having desired characteristics (conductive or semiconductive) cannot be attained.
Thus, research has been conducted into developing methods of growing a mixture of semiconducting carbon nanotubes and metallic carbon nanotubes and subsequently excluding unwanted carbon nanotubes.
A method using a surfactant, e.g., octadecylamine (ODA), has been suggested. In this method, after isolating chiral carbon nanotubes, the surfactant must be removed from the carbon nanotubes, resulting in a complicated process and low yield.
A method of isolating metallic carbon nanotubes has been proposed. The method includes dispersing carbon nanotubes in a solution and causing metallic carbon nanotubes to become attached to an electrode by electrophoresis. In this method, yield is low and it is difficult to produce the metallic carbon nanotubes having specific chirality in large quantities.
Another method of isolating carbon nanotubes having a desired chirality includes connecting both ends of carbon nanotubes to an electrode in parallel and applying a pulse voltage in a predetermined temperature range to remove carbon nanotubes having undesired chirality. However, in this method, it is difficult to connect the carbon nanotubes in parallel, and this method is not suitable for producing the desired carbon nanotubes in large quantities.